Heat exchanger

ABSTRACT

A heat exchanger includes exchange tubes formed from an aluminum extrudate and fins made of an aluminum bare material. The wall of each heat exchange tube is composed of a main body portion made of an Al alloy forming the aluminum extrudate, and a covering layer made of an Al—Si—Zn alloy and covering the main body portion. A diffusion layer containing Zn and Si diffused from the Al—Si—Zn alloy is formed in an outer surface layer portion of the main body portion. A low potential portion whose spontaneous potential is the lowest, and a high potential portion whose spontaneous potential is 60 mV or more higher than that of the low potential portion, are present within a range between an outermost surface of the wall and a deepest portion of the diffusion layer such that the low potential portion is located toward the outermost surface of the wall.

BACKGROUND OF THE INVENTION

The present invention relates to a heat exchanger. More particularly,the present invention relates to a heat exchanger which is used as acondenser for a car air conditioner mounted on a vehicle such as anautomobile.

In this specification and claims, the term “aluminum” encompassesaluminum alloys in addition to pure aluminum. Also, materialsrepresented by chemical symbols represent pure materials, and the term“Al alloy” means an aluminum alloy.

In this specification, the term “spontaneous potential” of a materialrefers to the electrode potential of the material within an acidic (pH:3) aqueous solution of 5% NaCl with respect to a saturated calomelelectrode (S.C.E.), which serves as a reference electrode.

A heat exchanger having the following structure has been widely knownand used as a condenser for a car air conditioner. The heat exchangerhas a plurality of flat heat exchange tubes formed from an aluminumextrudate, header tanks, corrugated aluminum fins, and aluminum sideplates. The flat heat exchange tubes are disposed at predeterminedintervals in their thickness direction such that they have the samelongitudinal direction and their width direction coincides with anair-flow direction. The header tanks are disposed at oppositelongitudinal ends of the heat exchange tubes such that theirlongitudinal directions coincide with the direction in which the heatexchange tubes are juxtaposed. Opposite ends of the heat exchange tubesare connected to the corresponding header tanks. Each of the fins isdisposed between adjacent heat exchange tubes or on the outer side ofthe heat exchange tube at each of opposite ends, and is brazed to thecorresponding heat exchange tube(s). The side plates are disposedoutward of the fins at opposite ends and are brazed to the correspondingfins. Each of the header tanks is composed of a tubular tank body formedof aluminum and closing members formed of aluminum. The tank body isformed by bending, into a tubular shape, an aluminum brazing sheethaving a brazing material layer on each of opposite sides thereof andbrazing opposite side edges of the sheet which are butted against eachother. The tank body has openings at opposite ends thereof. The closingmembers are brazed to the opposite ends of the tank body so as to closethe openings at the opposite ends. The tank body has a plurality of tubeinsertion holes elongated in the air-flow direction and spaced from oneanother in the longitudinal direction of the tank body. An end portionof each heat exchange tube is inserted into the corresponding tubeinsertion hole and is brazed to the tank body.

The present applicant has proposed a method of manufacturing theabove-described heat exchanger (see Japanese Patent ApplicationLaid-Open (kokai) No. 2014-238209). The proposed method includes stepsof: preparing heat exchange tubes and fins; adhering Zn powder and fluxpowder to outer surfaces of the heat exchange tubes; and brazing theheat exchange tubes and the corresponding fins and forming a Zn diffusedlayer in an outer surface layer portion of each of the heat exchangetubes. Each of the heat exchange tubes has a wall thickness of 200 μm orless and is formed from an aluminum extrudate made of an alloycontaining Mn in an amount of 0.2 to 0.3 massa, Cu in an amount of 0.05mass % or less, and Fe in an amount of 0.2 mass % or less, the balancebeing Al and unavoidable impurities. Each of the fins is formed from abrazing sheet composed of an aluminum core material, and a coatingmaterial formed of an aluminum brazing material and covering theopposite sides of the core material. In the step of adhering Zn powderand flux powder to the outer surfaces of the heat exchange tubes, adispersing liquid is prepared by mixedly dispersing flux powder, and Znpowder having an average particle size of 3 to 5 μm and a largestparticle size of less than 10 μm in a binder. The dispersing liquid isapplied to the outer surface of each of the heat exchange tubes, and theliquid component of the dispersing liquid is vaporized so as to adherethe Zn powder and the flux powder to the outer surface of each heatexchange tube such that the Zn powder adhesion amount becomes 1 to 3g/m², the flux powder adhesion amount becomes 15 g/m² or less, and theratio of the flux powder adhesion amount to the Zn powder adhesionamount (the flux powder adhesion amount/the Zn powder adhesion amount)becomes 1 or higher. In the step of brazing the heat exchange tubes andthe corresponding fins, the heat exchange tubes and the fins in anassembled condition are heated so as to braze the heat exchange tubesand the corresponding fins through utilization of the flux powderadhering to the outer surfaces of the heat exchange tubes and thecoating material of the fins and to melt the Zn powder adhering to theouter surfaces of the heat exchange tubes for diffusing Zn in outersurface layer portions of the heat exchange tubes so as to form Zndiffused layers in the respective outer surface layer portions of theheat exchange tubes.

In manufacture of the heat exchanger by the method described in thepublication, the heat exchange tubes and the corresponding fins arejoined by a brazing material melted out from the coating material of thebrazing sheet for forming the fins.

A conceivable method for further enhancing the corrosion resistance ofthe fins in the heat exchanger manufactured by the method described inthe publication is to use fins made of an aluminum bare material inplace of the fins formed from an aluminum brazing sheet. In this case,the described method may be modified such that, in addition to the Znpowder, Si powder is caused to adhere to the outer surfaces of the heatexchange tubes, and the heat exchange tubes and the corresponding finsare joined by a brazing material composed of Al contained in the Alalloy forming the aluminum extrudate from which the heat exchange tubesare formed, and Si of the Si powder caused, before joining, to adhere tothe surfaces of the heat exchange tubes.

However, in the heat exchanger manufactured by such a method, thecorrosion resistance of the heat exchange tubes may become insufficient.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above-describedproblem and to provide a heat exchanger in which heat exchange tubesexhibit excellent corrosion resistance.

A heat exchanger according to the present invention comprises aplurality of heat exchange tubes formed from an aluminum extrudate andfins made of an aluminum bare material, each disposed between adjacentheat exchange tubes, and joined to the corresponding heat exchange tubesby a brazing material. Each heat exchange tube has a wall composed of amain body portion made of an Al alloy forming the aluminum extrudate,and a covering layer made of an Al—Si—Zn alloy and covering an outersurface of the main body portion. A diffusion layer in which Zn and Sicontained in the Al—Si—Zn alloy forming the covering layer are diffusedis formed in an outer surface layer portion of the main body portion ofthe wall of each heat exchange tube. A low potential portion whosespontaneous potential is the lowest, and a high potential portion whosespontaneous potential is 60 mV or more higher than that of the lowpotential portion, are present within a range between an outermostsurface of the wall of each heat exchange tube and a deepest portion ofthe diffusion layer such that the low potential portion is locatedtoward the outermost surface of the wall.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the overall structure of acondenser for a car air conditioner to which a heat exchanger accordingto the present invention is applied;

FIG. 2 is an enlarged sectional view partially showing the wall of aheat exchange tube of the condenser of FIG. 1; and

FIG. 3 is a graph showing spontaneous potentials at different depthsfrom the wall outermost surface of a single heat exchange tube to whicha fin is brazed in an experimental example.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will next be described withreference to the drawings. In the embodiment, a heat exchanger accordingto the present invention is applied to a condenser for a car airconditioner.

FIG. 1 shows the overall structure of a condenser for a car airconditioner to which a heat exchanger according to the present inventionis applied, and FIG. 2 shows the structure of a main portion of thecondenser.

Notably, in the following description, the upper, lower, left-hand, andright-hand sides of FIG. 1 will be referred to as “upper,” “lower,”“left,” and “right,” respectively.

As shown in FIG. 1, a condenser 1 for a car air conditioner includes aplurality of flat heat exchange tubes 2 formed from an aluminumextrudate, corrugated fins 3 each formed of an aluminum bare material, apair of header tanks 4 and 5 formed of aluminum, and side plates 6formed from an aluminum brazing sheet. The heat exchange tubes 2 aredisposed at predetermined intervals in the vertical direction (thethickness direction of the heat exchange tubes 2) in such a manner thattheir longitudinal direction coincides with the left-right direction andtheir width direction coincides with an air-passing direction. Thecorrugated fins 3 are disposed between adjacent heat exchange tubes 2and on the outer sides of the uppermost and lowermost heat exchangetubes 2, and are brazed to the corresponding heat exchange tubes 2. Theheader tanks 4 and 5 are disposed at a predetermined interval in theleft-right direction in such a manner that their longitudinal directioncoincides with the vertical direction (the direction in which the heatexchange tubes 2 are juxtaposed). Left and right end portions of theheat exchange tubes 2 are connected to the header tanks 4 and 5. Theside plates 6 are disposed on the outer sides of the uppermost andlowermost corrugated fins 3, and are brazed to the correspondingcorrugated fins 3. Air flows in a direction indicated by an arrow W inFIG. 1.

The left header tank 4 is divided by a partition plate 7 into upper andlower header sections 4 a and 4 b, at a position higher than the centerof the left header tank 4 in the height direction. The right header tank5 is divided by another partition plate 7 into upper and lower headersections 5 a and 5 b, at a position lower than the center of the rightheader tank 5 in the height direction. A refrigerant inlet (not shown)is formed at the upper header section 4 a of the left header tank 4, andan aluminum inlet member 8 having an inflow passage 8 a communicatingwith the refrigerant inlet is brazed to the upper header section 4 a. Arefrigerant outlet (not shown) is formed at the lower header section 5 bof the right header tank 5, and an aluminum outlet member 9 having anoutflow passage 9 a communicating with the refrigerant outlet is brazedto the lower header section 5 b. Refrigerant having flowed into theupper header section 4 a of the left header tank 4 through the inflowpassage 8 a of the inlet member 8 flows rightward within the heatexchange tubes 2 located above the partition plate 7 of the left headertank 4, and flows into an upper portion of the upper header section 5 aof the right header tank 5. The refrigerant then flows downward withinthe upper header section 5 a, flows leftward within the heat exchangetubes 2 whose vertical positions are located between the partition plate7 of the left header tank 4 and the partition plate 7 of the rightheader tank 5, and flows into an upper portion of the lower headersection 4 b of the left header tank 4. The refrigerant then flowsdownward within the lower header section 4 b, flows rightward within theheat exchange tubes 2 located below the partition plate 7 of the rightheader tank 5, and flows into the lower header section 5 b of the rightheader tank 5. The refrigerant then flows to the outside of thecondenser 1 through the outflow passage 9 a of the outlet member 9.

Each of the left and right header tanks 4 and 5 is formed from analuminum pipe having a brazing material layer on at least an outersurface thereof; for example, a tubular member formed by bending analuminum brazing sheet having a brazing material layer on each ofopposite sides thereof into a tubular shape and brazing side edgesthereof which overlap each other. Each of the left and right headertanks 4 and 5 is composed of a tank body 11 having a plurality of tubeinsertion holes elongated in the air-flow direction, and aluminumclosing members 12 brazed to the opposite ends of the tank body 11 so asto close the openings at the opposite ends. A detailed illustration ofthe tank body 11 is omitted. Also, the tank body 11 may be formed from atubular aluminum extrudate having a brazing material thermally sprayedto an outer circumferential surface thereof.

Preferably, each heat exchange tube 2 is formed from an extrudate formedof, for example, an Al alloy containing Cu in an amount of 0.4 to 0.5mass % and Mn in an amount of 0.1 to 0.3 mass %, the balance being Aland unavoidable impurities. The Al alloy is usually used for forming aheat exchange tube formed from an extrudate.

As shown in FIG. 2, each heat exchange tube 2 has a wall 30 composed ofa main body portion 31 and a covering layer 32. The main body portion 31is made of an Al alloy forming the aluminum extrudate. The coveringlayer 32 is made of an Al—Si—Zn alloy and covers the outer surface ofthe main body portion 31. A diffusion layer 33 is formed in an outersurface layer portion of the main body portion 31 of the wall 30 as aresult of diffusion of Zn and Si contained in the Al—Si—Zn alloy formingthe covering layer 32.

Preferably, the wall 30 of each heat exchange tube 2 has a thickness of200 μm or less. The thickness of the wall 30 of the heat exchange tube 2may not be uniform, but may differ locally. The expression “the wall 30has a thickness of 200 μm or less” means that a thickest portion of thewall 30 has a thickness of 200 μm or less.

A low potential portion whose spontaneous potential is the lowest, and ahigh potential portion whose spontaneous potential is 60 mV or morehigher than that of the low potential portion, are present within arange between an outermost surface 34 of the wall 30 of each heatexchange tube 2 and a deepest portion 35 of the diffusion layer 33 suchthat the low potential portion is located toward the outermost surface34 of the wall 30. For example, within the range between the outermostsurface 34 of the wall 30 and the deepest portion 35 of the diffusionlayer 33, the spontaneous potential of the wall 30 lowers gradually fromthe outermost surface 34 of the wall 30 toward the main body portion 31up to the low potential portion, and the spontaneous potential of thewall 30 increases from the low potential portion toward the main bodyportion 31 up to the high potential portion.

Cu contained in the alloy forming the aluminum extrudate-made heatexchange tubes 2 has the effect of improving the corrosion resistance ofthe main body portion 31 of each heat exchange tube 2. However, at a Cucontent of less than 0.4 massa, the effect is not yielded. At a Cucontent in excess of 0.5 massa, the sacrificial corrosion effect of thediffusion layer 33 for the main body portion 31 deteriorates. That is,the diffusion layer 33 in which Zn is diffused has the effect oflowering the spontaneous potential of the diffusion layer 33 and isformed so as to serve as a sacrificial corrosion layer for the main bodyportion 31. However, at a Cu content in excess of 0.5 mass %, the effectof Zn becomes insufficient, resulting in a failure to sufficiently lowerthe spontaneous potential of the diffusion layer 33. Therefore,preferably, the Cu content is 0.4 to 0.5 mass %. Also, Mn contained inthe alloy forming the aluminum extrudate-made heat exchange tubes 2 hasthe effect of improving the strength of the heat exchange tubes 2.However, at an Mn content of less than 0.1 mass %, the effect is notyielded. At an Mn content in excess of 0.3 mass %, extrusion workabilitydeteriorates. Therefore, preferably, the Mn content is 0.1 to 0.3 mass%.

In some cases, the alloy forming the aluminum extrudate-made heatexchange tubes 2 contains, as unavoidable impurities, Si in an amount of0.2 mass % or less, Fe in an amount of 0.2 mass % or less, Mg in anamount of 0.05 mass % or less, Cr in an amount of 0.05 mass % or less,Zn in an amount of 0.05 mass % or less, and Ti in an amount of 0.05 mass% or less. In some cases, the content of these unavoidable impurities iszero. At excessively high Si and Fe contents, the corrosion resistanceof the heat exchange tube 2 deteriorates. At an excessively high Zncontent, the spontaneous potential of the heat exchange tube 2 lowers,resulting in a change in potential balance in relation to peripheralcomponents. At an excessively high Ti content, the cost increases.Further, in some cases, unavoidable impurities other than Si, Fe, Mg,Cr, Zn, and Ti are contained such that individual contents are 0.05 mass% or less (including zero mass %) and such that the total content is0.15 mass % or less.

Preferably, each of the corrugated fins 3 is formed of, for example, anAl alloy containing Mn in an amount of 1.0 to 1.5 mass % and Zn in anamount of 1.2 to 1.8 mass %, the balance being Al and unavoidableimpurities. The Al alloy forming the corrugated fins 3 is an ordinaryalloy used as a bare material for forming fins.

Mn contained in the alloy forming the corrugated fins 3 has the effectof improving the strength of the corrugated fins 3. However, at an Mncontent of less than 1.0 mass %, the effect is not yielded. At an Mncontent in excess of 1.5 mass %, workability deteriorates. Therefore,the Mn content is set to 1.0 to 1.5 mass %.

Zn contained in the alloy forming the corrugated fins 3 has the effectof appropriately maintaining potential balance with the heat exchangetubes 2. However, at a Zn content of less than 1.2 mass %, the effect isnot yielded. At a Zn content in excess of 1.8 massa, corrosion of thecorrugated fins 3 becomes intensive. Therefore, the Zn content is set to1.2 to 1.8 mass %.

In some cases, the Al alloy forming the corrugated fins 3 contains, asunavoidable impurities, Si in an amount of 0.6 mass % or less, Fe in anamount of 0.5 mass % or less, Cu in an amount of 0.05 mass % or less,and Cr in an amount of 0.12 mass % or less. In some cases, the contentof these unavoidable impurities is zero. At excessively high Si, Fe, andCu contents, the corrosion rate of the corrugated fins 3 increases.Further, in some cases, unavoidable impurities other than Si, Fe, Cu,and Cr are contained such that individual contents are 0.05 mass % orless (including zero mass %) and such that the total content is 0.15mass % or less.

The condenser 1 is manufactured by a method described below.

First, there are prepared the heat exchange tubes 2 formed from anextrudate made of the Al alloy described above, the corrugated fins 3made of the Al alloy described above, the side plates 6, the partitionplates 7, a pair of tubular aluminum header tank body intermediateshaving a brazing material layer on at least the outer surfaces thereof,the closing members 12, the inlet member 8, and the outlet member 9. Theheader tank body intermediates have a plurality of tube insertion holesformed therein.

A dispersing liquid is prepared by mixedly dispersing flux powder, Znpowder, and Si powder in a binder. The Zn powder has an average particlesize of 3 to 5 μm and a maximum particle size of less than 10 μm. The Sipowder has an average particle size of 2 to 6 μm and a maximum particlesize of less than 10 μm. The flux powder is of, for example,fluoride-based noncorrosive flux containing a mixture of KAlF₄ and KAlF₅as a main component. The binder is, for example, a solution prepared bydissolving acrylic resin in 3-methoxy-3-methyl-1-butanol. Notably, inorder to adjust the viscosity of the binder, a diluent of, for example,3-methoxy-3-methyl-1-butanol is added to the dispersing liquid.

Next, the dispersing liquid is applied to the outer surface of each heatexchange tube 2, and the liquid component of the dispersing liquid isvaporized so as to cause the Zn powder, the Si powder, and the fluxpowder to adhere to the outer surface of each heat exchange tube suchthat the Zn powder adhesion amount becomes 4 to 6 g/m², the Si powderadhesion amount becomes 3 to 6 g/m², and the flux powder adhesion amountbecomes 6 to 24 g/m². A method of causing the Zn powder, the Si powder,and the flux powder to adhere to the outer surface of each heat exchangetube 2 is as follows: the dispersing liquid is applied to the outersurface of each heat exchange tube 2 by a spraying process, andsubsequently, each heat exchange tube 2 is dried through application ofheat for vaporizing the liquid component of the dispersing liquid,thereby causing the Zn powder, the Si powder, and the flux powder toadhere to the outer surface of each heat exchange tube 2; alternatively,the dispersing liquid is applied to the preheated outer surface of eachheat exchange tube 2 by a roll coating process, and subsequently, eachheat exchange tube 2 is dried through application of heat for vaporizingthe liquid component of the dispersing liquid, thereby causing the Znpowder, the Si powder, and the flux powder to adhere to the outersurface of each heat exchange tube 2.

As a result of adhesion of the Zn powder, the Si powder, and the fluxpowder to the outer surface of each heat exchange tube 2, a flux powderlayer containing the Zn powder and the Si powder is formed on the outersurface of the heat exchange tube 2. In the flux powder layer, the Znpowder and the Si powder are uniformly dispersed.

Next, the paired header tank body intermediates having the tubeinsertion holes formed therein are disposed at a predetermined interval;the closing members 12 are disposed at the opposite ends of both headertank body intermediates; and the partition plates 7 are disposed in therespective header tank body intermediates. The heat exchange tubes 2 andthe fins 3 are alternately disposed, and opposite end portions of theheat exchange tubes 2 are inserted into the corresponding tube insertionholes of the header tank body intermediates. The side plates 6 aredisposed outward of the fins 3 at opposite ends, and the inlet member 8and the outlet member 9 are disposed in place.

Next, header tank intermediates composed of the header tank bodyintermediates, the closing members 12, and the partition plates 7, theheat exchange tubes 2, the fins 3, the side plates 6, the inlet member8, and the outlet member 9 are temporarily fixed together, therebyyielding a provisional assembly.

Next, the provisional assembly is placed in a brazing furnace and isheated to a predetermined temperature within the brazing furnace.Notably, flux is applied beforehand to components other than the heatexchange tubes 2 as needed by a publicly known method such as brushing.

In the course of increasing the temperature of the provisional assembly,first, the flux powder of the flux powder layer melts, thereby breakingoxide films on the outer surfaces of the heat exchange tubes 2, oxidefilms on the outer surfaces of the corrugated fins 3, oxide films onparticle surfaces of the Si powder, and oxide films on particle surfacesof the Zn powder. Next, Si and Zn diffuse in the outer surface layerportions of the heat exchange tubes 2 to thereby form a brazing materialof Al—Si—Zn alloy having a low melting point in the outer surface layerportions of the heat exchange tubes 2. The brazing material brazes theheat exchange tubes 2 and the corrugated fins 3. The remainder of thebrazing material not used for brazing becomes the covering layers 32,and Zn and Si contained in the Al—Si—Zn alloy of the covering layers 32diffuse to thereby form the diffusion layers 33. At the same time,molten flux on the outer surfaces of the heat exchange tubes 2 flows andspreads. Also, molten Zn flows and spreads; as a result, Zn diffuses inthe outer surface layer portions of the heat exchange tubes 2, therebyforming Zn diffused layers. By this procedure, the condenser 1 ismanufactured.

In each heat exchange tube 2 of the thus-manufactured condenser 1, asdescribed above, the wall 30 includes the main body portion 31, thecovering layer 32, and the diffusion layer 33 formed in the outersurface layer portion of the main body portion 31. A low potentialportion whose spontaneous potential is the lowest, and a high potentialportion whose spontaneous potential is 60 mV or more higher than that ofthe low potential portion, are present within the range between theoutermost surface 34 of the wall 30 and the deepest portion 35 of thediffusion layer 33 such that the low potential portion is located towardthe outermost surface 34 of the wall 30.

On the basis of the results of a test which will be described next, thefollowing limitation is imposed on each heat exchange tube 2: a lowpotential portion whose spontaneous potential is the lowest, and a highpotential portion whose spontaneous potential is 60 mV or more higherthan that of the low potential portion, are present within the rangebetween the outermost surface 34 of the wall 30 and the deepest portion35 of the diffusion layer 33 such that the low potential portion islocated toward the outermost surface 34 of the wall 30.

There were prepared heat exchange tubes formed from an extrudate made ofan Al alloy containing Cu in an amount of 0.42 massa, Mn in an amount of0.16 mass %, Si in an amount of 0.12 mass %, Fe in an amount of 0.11mass %, and Ti in an amount of 0.01 mass %, the balance being Al andunavoidable impurities, and corrugated fins formed from a bare materialof an Al alloy containing Si in an amount of 0.77 mass %, Fe in anamount of 0.24 mass %, Mn in an amount of 1.68 mass %, Zn in an amountof 1.60 mass %, and Zr in an amount of 0.11 mass %, the balance being Aland unavoidable impurities. The Al alloy forming the heat exchange tubescontains unavoidable impurities other than Si, Fe, and Ti such thatindividual contents are 0.05 mass % or less and such that the totalcontent is 0.15 mass % or less. Each heat exchange tube has a wallthickness of 180 μm, and each corrugated fin has a thickness of 70 μm.

Further, there were prepared fluoride-based noncorrosive flux powdercontaining a mixture of KAlF₄ and KAlF₅ (KAlF₅ content in the mixture:10 to 40 mass %) in an amount of 90 mass % or more, Zn powder (zincoxide accounts for 5 mass % of the total weight of the Zn powder) havingan average particle size of 3 to 5 μm and a maximum particle size ofless than 10 μm, Si powder having an average particle size of 2 to 6 μmand a maximum particle size of less than 10 μm, a binder in the form ofa solution prepared by dissolving acrylic resin in3-methoxy-3-methyl-1-butanol, and a diluent of3-methoxy-3-methyl-1-butanol. The Zn powder, the Si powder, and thenoncorrosive flux powder were mixedly dispersed in the binder and thediluent, thereby yielding a dispersing liquid. The weight ratios of allthe components of the dispersing liquid are as follows: Zn powder: 14.1parts by weight; Si powder: 10.6 parts by weight; noncorrosive fluxpowder: 21.1 parts by weight; binder: 9.2 parts by weight; and diluent:45.0 parts by weight.

Next, after the dispersing liquid was applied by spraying to the outersurfaces of the heat exchange tubes, the heat exchange tubes were driedwithin a drying machine for vaporizing the liquid component of thedispersing liquid so as to cause the Zn powder, the Si powder, and theflux powder to adhere to the outer surfaces of the heat exchange tubessuch that the Zn powder adhesion amount becomes 4 to 6 g/m², the Sipowder adhesion amount becomes 3 to 6 g/m², and the flux powder adhesionamount becomes 24 g/m² or less.

Subsequently, the heat exchange tubes and the corrugated fins werealternately stacked and were heated in a nitrogen gas atmosphere withina furnace for brazing the heat exchange tubes and the corrugated fins.The heat exchange tubes and the corrugated fins were heated for 6.3minutes such that the heat exchange tubes had a substantial temperatureof 580° C. or higher and a maximum temperature of 600.7° C.

One heat exchange tube was cut off from a brazed assembly of the heatexchange tubes and the fins, and the spontaneous potential was measuredat different depths from the outermost surface 34 of the wall 30. FIG. 3shows the results of the measurement. The thickness of the wall 30 was180 μm. As shown in FIG. 3, a low potential portion whose spontaneouspotential was the lowest within the range between the outermost surface34 of the wall 30 and the deepest portion 35 of the diffusion layer 33was located at the position indicated by straight line A; i.e., at adepth of 7 μm from the outermost surface 34. The deepest portion 35 ofthe diffusion layer 33 was located at a depth of 100 μm from theoutermost surface 34 of the wall 30. It is found from the results shownin FIG. 3 that a low potential portion whose spontaneous potential isthe lowest, and a high potential portion whose spontaneous potential is60 mV or more higher than that of the low potential portion, are presentwithin the range between the outermost surface 34 of the wall 30 and thedeepest portion 35 of the diffusion layer 33 such that the low potentialportion is located toward the outermost surface 34 of the wall 30.

Further, after the CCT test was carried out on the brazed assembly ofthe heat exchange tubes and the fins for 240 days, one heat exchangetube was cut off, and the depth of corrosion of the wall 30 of the heatexchange tube from the outermost surface 34 was measured. The measuredmaximum corrosion depth was 53.0 μm, indicating that corrosion stoppedat the high potential portion present in the diffusion layer 33. Also, aremaining portion of the wall 30 of the heat exchange tube after the CCTtest has a thickness of 100 μm or more, indicating that the heatexchange tube has sufficient corrosion resistance.

On the basis of the above-mentioned test results, the followinglimitation was imposed on each heat exchange tube 2: a low potentialportion whose spontaneous potential is the lowest, and a high potentialportion whose spontaneous potential is 60 mV or more higher than that ofthe low potential portion, are present within the range between theoutermost surface 34 of the wall 30 and the deepest portion 35 of thediffusion layer 33 such that the low potential portion is located towardthe outermost surface 34 of the wall 30.

The present invention comprises the following modes.

1) A heat exchanger comprising a plurality of heat exchange tubes formedfrom an aluminum extrudate; and fins made of an aluminum bare material,each disposed between adjacent heat exchange tubes, and joined to thecorresponding heat exchange tubes by a brazing material, wherein eachheat exchange tube has a wall composed of a main body portion made of anAl alloy forming the aluminum extrudate, and a covering layer made of anAl—Si—Zn alloy and covering an outer surface of the main body portion; adiffusion layer in which Zn and Si contained in the Al—Si—Zn alloyforming the covering layer are diffused is formed in an outer surfacelayer portion of the main body portion of the wall of each heat exchangetube; and a low potential portion whose spontaneous potential is thelowest, and a high potential portion whose spontaneous potential is 60mV or more higher than that of the low potential portion, are presentwithin a range between an outermost surface of the wall of each heatexchange tube and a deepest portion of the diffusion layer such that thelow potential portion is located toward the outermost surface of thewall.

2) The heat exchanger described in par. 1), wherein within the rangebetween the outermost surface of the wall of each heat exchange tube andthe deepest portion of the diffusion layer, the spontaneous potential ofthe wall lowers from the outermost surface of the wall toward the mainbody portion up to the low potential portion, and the spontaneouspotential of the wall increases from the low potential portion towardthe main body portion up to the high potential portion.

3) The heat exchanger described in par. 1) or 2), wherein the brazingmaterial for joining the heat exchange tubes and the corresponding finsis composed of Al contained in the Al alloy forming the aluminumextrudate, and Si of Si powder caused, before joining, to adhere tosurfaces of the heat exchange tubes.

4) The heat exchanger described in any of pars. 1) to 3), wherein theAl—Si—Zn alloy serving as the covering layer of each heat exchange tubeis composed of Al contained in the Al alloy forming the aluminumextrudate, Si of Si powder caused, before joining, to adhere to thesurface of the heat exchange tube, and Zn of Zn powder caused, beforejoining, to adhere to the surface of the heat exchange tube.

According to the heat exchangers of pars. 1) to 4), the fins are formedof an aluminum bare material and thus exhibit an improved corrosionresistance as compared with the case of a heat exchanger having finsformed of an aluminum brazing sheet.

Also, each heat exchange tube has a wall composed of a main body portionmade of an Al alloy forming the aluminum extrudate, and a covering layermade of an Al—Si—Zn alloy and covering an outer surface of the main bodyportion; a diffusion layer in which Zn and Si contained in the Al—Si—Znalloy forming the covering layer are diffused is formed in an outersurface layer portion of the main body portion of each heat exchangetube; and a low potential portion whose spontaneous potential is thelowest, and a high potential portion whose spontaneous potential is 60mV or more higher than that of the low potential portion, are presentwithin the range between an outermost surface of the wall of each heatexchange tube and a deepest portion of the diffusion layer such that thelow potential portion is located toward the outermost surface of thewall. Therefore, corrosion from the outer surface of the wall of eachheat exchange tube stops at the high potential portion. Accordingly, thecorrosion depth can be made shallow, whereby the corrosion resistance ofthe heat exchange tubes is improved. As a result, the thickness of thewall of each heat exchange tube can be decreased, whereby the weight ofthe heat exchange tubes can be decreased, and thus, the weight of theheat exchanger using the heat exchange tubes can be decreased.

What is claimed is:
 1. A heat exchanger comprising: a plurality of heatexchange tubes formed from an aluminum extrudate; and fins made of analuminum bare material, each disposed between adjacent heat exchangetubes, and joined to the corresponding heat exchange tubes by a brazingmaterial, wherein each heat exchange tube has a wall composed of a mainbody portion made of an Al alloy forming the aluminum extrudate, and acovering layer made of an Al—Si—Zn alloy and covering an outer surfaceof the main body portion; a diffusion layer in which Zn and Si containedin the Al—Si—Zn alloy forming the covering layer are diffused is formedin an outer surface layer portion of the main body portion of the wallof each heat exchange tube; and a low potential portion whosespontaneous potential is the lowest, and a high potential portion whosespontaneous potential is 60 mV or more higher than that of the lowpotential portion, are present within a range between an outermostsurface of the wall of each heat exchange tube and a deepest portion ofthe diffusion layer such that the low potential portion is locatedtoward the outermost surface of the wall.
 2. The heat exchangeraccording to claim 1, wherein within the range between the outermostsurface of the wall of each heat exchange tube and the deepest portionof the diffusion layer, the spontaneous potential of the wall lowersfrom the outermost surface of the wall toward the main body portion upto the low potential portion, and the spontaneous potential of the wallincreases from the low potential portion toward the main body portion upto the high potential portion.
 3. The heat exchanger according to claim1, wherein the brazing material for joining the heat exchange tubes andthe corresponding fins is composed of Al contained in the Al alloyforming the aluminum extrudate, and Si of Si powder caused, beforejoining, to adhere to surfaces of the heat exchange tubes.
 4. The heatexchanger according to claim 1, wherein the Al—Si—Zn alloy serving asthe covering layer of each heat exchange tube is composed of Alcontained in the Al alloy forming the aluminum extrudate, Si of Sipowder caused, before joining, to adhere to the surface of the heatexchange tube, and Zn of Zn powder caused, before joining, to adhere tothe surface of the heat exchange tube.